As an example of equipment with a system for cooling a high temperature strip material, a hot dip galvanizing system is shown in FIG. 3. This system comprises a hot dip galvanizing tank 60, a heater 71, a soaking device 72, and a mist cooler 80 as a cooling device.
According to the above system, a steel strip 50 is galvanized in the hot dip galvanizing tank 60, moved vertically upward, and heated with the heater 71 to alloy the zinc with the steel. The alloyed steel strip 50 is soaked over its entire width by means of the soaking device 72. This steel strip 50 traveling in a cooling zone C is cooled with the mist cooler 80 from 520.degree. C. to 200.degree. C., and carried horizontally by a deflector roll 90.
The mist cooler 80 is composed of mist sprayers 81 disposed in opposing positions at both sides of the ascending steel strip 50. Each mist sprayer 81 comprises water supply pipes 82 and air supply pipes 83 arranged vertically in rows such that each air supply pipe 83 is mounted inside each water supply pipe 82 in a double-pipe configuration. Each water supply pipe 82 has many nozzle holes made along the width of the steel strip 50, and each air supply pipe 83 has many nozzle holes made along the width of the steel strip 50. The mist cooler 80 forms mists 86 from water 84 in the water supply pipes 82 by jetting air 85 through the nozzles of the air supply pipes 83, and directs the mists 86 toward the surfaces of the steel strip 50 to cool it.
With the foregoing mist cooler 80, mists 86 with a constant water volume density were sprayed on both sides of the steel strip 50 throughout the cooling zone C to cool the steel strip 50. At a site in the cooling zone C where the temperature of the steel strip 50 was about 350.degree. C. or lower (i.e., an upper portion of the cooling zone C), however, the mists 86 adhering to the surfaces of the steel strip 50 underwent transition boiling, rapidly cooling the steel strip 50. Transition boiling refers, in terms of water, to a phenomenon involving transition from a state of cooling with water vapor to a state of direct cooling with water, or to a state of cooling with a mixture of water and water vapor. This phenomenon takes place at about 350.degree. C. Thus, nonuniform temperature distribution of the steel strip 50 was liable to occur, thereby deforming the steel strip 50, resulting in its malformation.